The present invention relates to data compression of sound data, and more particularly to the data compression of sound data utilized in digital sampling keyboard instruments.
Since the introduction of digital sampling keyboard instruments, the desire to compress sound data into smaller memories without compromising sound quality is ever increasing. In recent years, limiting bit resolution (8 to 12 bits) and sample rates (less than 44.1 Khz) have been two common methods to reduce memory size. But since the introduction of the compact disk (CD), resolution less than 16-bit and 44.1 Khz has largely been deemed unacceptable.
Another common approach, looping, involves repeating a section of data during the time a key is depressed. Two common types of loops are single period forwards loops and cross-faded forwards loops (see FIGS. 1 and 2). Single period (or single cycle) loops characteristically sound quite static, as only one period is repeated. They work best on solo instruments with non-complex harmonic structures. Longer loops, on the other hand, are required for ensemble sounds and harmonically complex solo sounds. Often, the sound data must be processed to avoid pops in the loop. This process is called cross/fade looping. Portions of the sound at the loop start and end points are faded in and out of the loop. Obviously, the longer, cross-faded loop contains more dynamics than a single-cycle loop. However, some lower frequency phase cancellation occurs as a result.
The start point of a cross/faded loop must begin after the attack phase of the sound has passed and the sound becomes more stable. The problem here is that it often takes a while for a sound to become stable. If a loop is started too close to the attack, poor loops result due to large fluctuations in phase and amplitude, and there is a high risk of attack data becoming part of the loop.
Yet another method for reducing memory is to simply take fewer samples of a given instrument across the keyboard. A single sample of a violin will use less memory than one that has been sampled every half octave. The problem here is that the realism of the sound disintegrates rapidly when too few samples are used to represent a fixed formant instrument.